1. Field of the Invention
The present invention relates to a communication device and method for transmitting a sequence of data applied thereto at a certain transmission rate over a transmission line (or channel) that accommodates a larger transmission rate.
2. Description of the Prior Art
Recent years have seen the increasing use of image communication with the increasing capacity of networks and as image processing techniques grow more sophisticated. Especially, as for images, an image compression technique called MPEG-2, which is defined by International Standard ISO/IEC (International Organization for Standardization/International Electrotechnical Commission) 13,818, springs into wide use. There are two kinds of transmission formats that conform to MPEG-2; program streams (or PS) and transport streams (or TS). Transport streams constructed of 188-byte fixed-length packets come into wide use in transmission media such as satellite digital broadcasting and ground wave digital broadcasting.
In communications, there exist asynchronous transfer mode (or ATM), local area networks (or LAN), dedicated (or leased) lines such as a T1 network and a digital signal level 3 (or DS3) network defined by American National Standards Institute or ANSI, and so on, as transmission lines. How to bear MPEG-2 TS streams over such a communication network (this is referred to as mapping) is an important issue in the use of the communication network for digital TV broadcasting.
Referring next to FIG. 17, there is illustrated a block diagram showing the structure of a prior art communication device and method for mapping MPEG-2 TS streams into DS3 frames and for transmitting them over a DS3 network, which is widely used mainly in North America. In the figure, reference numeral 1 denotes a communication device, numeral 2 denotes an MPEG-2 TS generating device, numeral 19 denotes an MPEG over ATM mapping unit, numeral 20 denotes a physical layer convergence protocol (or PLCP) framing unit, numeral 7 denotes a multiplexer, numeral 8 denotes a line coding unit, and numeral 10 denotes a system control unit for controlling the whole of the communication device 1.
FIG. 18 is a diagram showing a method of mapping MPEG-2 TS streams into ATM cells, and FIG. 19 is a diagram showing the structure of a PLCP frame that is constructed when mapping ATM cells into the DS3 network channel.
In operation, MPEG-2 TS generating device 2 generates. MPEG-2 TS data constructed of 188-byte fixed-length packets based on information from a video source, and then sends out the MPEG-2 TS data to the communication device 1. The MPEG-2 TS data is then furnished to the MPEG over ATM mapping unit 19.
The MPEG over ATM mapping unit 19 divides each 188-byte MPEG-2 TS into four or more parts and then maps each of them into the 48-byte payload of each of four or more ATM cells. This mapping method is defined by Audio/Visual Multimedia Services: Video on Demand v1.1, which is a standard established by ATM Forum that is the ATM LAN standardization group in U.S.A. According to the standard, 384 bytes of data obtained by adding an 8-byte trailer defined by the ATM adaptation layer type 5 (or AAL5) to two MPEG-2 TS packets of 376 bytes is mapped into the payload of eight ATM cells.
Those ATM cells into which MPEG-2 TS data has been mapped in the above-mentioned way are then furnished to the PLCP framing unit 20. The PLCP framing unit 20 performs a PLCP framing operation, as shown in FIG. 19, on the received ATM cells. The PLCP framing method of mapping ATM cells into frames to be transmitted over the DS3 network channel is defined by ATM User-Network Interface Specification V3.1 which is a standard established by ATM Forum. According to the standard, twelve ATM cells are mapped into one PLCP frame to be transmitted at a frame rate of 125 microseconds. The difference between the transmission rate for MPEG-2 TS and that for the DS3 network channel can be adjusted by inserting an idle cell as an ATM cell into each frame.
The multiplexer 7 then multiplexes PLCP frames generated by the PLCP framing unit 20 and a group of DS3 headers generated by the DS3 header generating unit 4 to generate DS3 frames, and then sends out them to the line coding unit 8. Each DS3 frame is 4760 bits in length. And, each DS3 frame has such a frame structure that it is divided into seven 680-bit sub-frames. Furthermore, each of the plurality of sub-frames is constructed of eight 85-bit blocks each of which includes a one-bit DS3 header at the head thereof. The transmission rate for the DS3 network channel is 44.736 Mb/s.
The line coding unit 8 performs a line coding operation on the DS3 frames generated by the multiplexer 7, and then sends out the line coded DS3 frames over the DS3 network channel. As the line coding, a kind of alternate mark inversion (or AMI) coding called bipolar with three zero substitution or B3ZS coding is used. The system control unit 10 controls the whole of the communication device 1.
As previously mentioned, the prior art communication device 1 can map MPEG-2 TS streams into DS3 frames to be transmitted over the DS3 network channel so as to transmit the MPEG-2 TS streams by combining the MPEG over ATM technique with the PLCP framing technique.
A problem with the prior art communication method and device constructed above is that since much time is spent in performing the two steps of mapping MPEG-2 TS data into ATM cells first, and then mapping the ATM cells into PLCP frames, in order to transmit the MPEG-2 TS data over the DS3 network channel, the delay is increased, and the complicated format processing required for carrying out the two steps increase the size of circuitry that implements the communication device.
In addition, mapping MPEG-2 TS data into ATM cells and further mapping them into PCLP frames can increase the overhead for the system, and therefore the transmission rate for MPEG-2 TS data that the network can accommodate is restricted with respect to the transmission band of the network. In other words, the utilization factor of the network is reduced. Since the difference between the transmission rate for MPEG-2 TS and that for the DS3 network channel must be adjusted by inserting an idle cell as an ATM cell into each PLCP frame, MPEG-2 TS data cannot be distributed uniformly with respect to time during the transmission over the DS3 network channel. In other words, there easily cause variations in the transmission of MPEG-2 TS data, such as a burst of data transmission, over the DS3 network channel.
The present invention is made to overcome the above problems. It is therefore an object of the present invention to provide a communication method and device for mapping a sequence of data, such as MPEG-2 TS data, into frames to be transmitted over a network, such as a DS3 network, the method and device being capable of reducing the delay caused by the mapping, improving the utilization factor of the network, reducing variations in the data rate that can occur during the transmission of MPEG-2 TS data over the network, and reducing the size of the circuitry that implements the communication device.
In accordance with one aspect of the present invention, there is provided a communication method for transmitting a sequence of data, which is transmitted thereto at a given transmission rate, at a desired transmission rate greater than the former transmission rate, the method comprising the steps of: in order to map the sequence of data into a plurality of frames in a predetermined form, each of which consists of a plurality of blocks, assigning a predetermined amount of data to a data transmission area in each of the plurality of blocks included in each frame so that the sequence of data is nearly-uniformly arranged over the plurality of blocks included in each frame; providing a stuff area for a predetermined one of the plurality of blocks included in each frame and determining whether or not to map part of the sequence of data into the stuff area, so as to transmit the plurality of frames at the desired transmission rate; mapping either part of the sequence of data or null data into the stuff area according to a determination done in the determining step; and providing a stuff present identifier indicating whether or not part of the sequence of data is mapped into the stuff area for each frame.
In accordance with a preferred embodiment of the present invention, the stuff present identifier providing step is the step of providing an odd number of stuff present identifiers each indicating whether or not part of the sequence of data is mapped into the stuff area for each frame so that they are distributed in each frame.
In accordance with another preferred embodiment of the present invention, the communication method further comprises the step of adding an error-detecting code to the stuff present identifier.
In accordance with another preferred embodiment of the present invention, the communication method further comprises the step of adding an error-correcting code to the stuff present identifier.
In accordance with another preferred embodiment of the present invention, the communication method further comprises the step of mapping supplemental information on the sequence of data into free space of the data transmission area of at least one of the plurality of blocks included in each frame. Preferably, information on the transmission rate at which the sequence of data to be mapped into the plurality of frames in the predetermined form is transmitted is mapped, as the supplemental information, into the free space. As an alternative, information on the status or characteristics of communication upon transmission of the plurality of frames can be mapped, as the supplemental information, into the free space.
In accordance with another preferred embodiment of the present invention, both the data transmission area of each of the plurality of blocks into which the sequence of data is mapped and the stuff area included in each frame are defined in units of one byte so that the sequence of data is separately mapped into the data transmission area of each of the plurality of blocks included in each frame in units of one byte in the same way that the sequence of data to be mapped into the plurality of frames can be separated in units of one byte.
In accordance with another aspect of the present invention, there is provided a communication device for transmitting a sequence of data, which is transmitted thereto at a given transmission rate, at a desired transmission rate greater than the former transmission rate over a transmission line, the device comprising: a mapping unit, in order to map the sequence of data into a plurality of frames in a predetermined form, each of which consists of a plurality of blocks, for assigning a predetermined amount of data to a data transmission area in each of the plurality of blocks included in each frame so that the sequence of data is nearly-uniformly arranged over the plurality of blocks included in each frame; and a control unit for controlling the mapping unit in such a manner that the mapping unit provides a stuff area for a predetermined one of the plurality of blocks included in each frame and determines whether or not to map part of the sequence of data into the stuff area, so as to transmit the plurality, of frames at the desired transmission rate, then maps either part of the sequence of data or null data into the stuff area according to a determination result, and provides a stuff present identifier indicating whether or not part of the sequence of data is mapped into the stuff area for each frame.
In accordance with a preferred embodiment of the present invention, the control unit controls the mapping unit in such a manner that the mapping unit provides an odd number of stuff present identifiers each indicating whether or not part of the sequence of data is mapped into the stuff area for each frame so that they are distributed in each frame.
In accordance with another preferred embodiment of the present invention, the control unit controls the mapping unit in such a manner that the mapping unit adds an error-detecting code to the stuff present identifier.
In accordance with another preferred embodiment of the present invention, the control unit controls the mapping unit in such a manner that the mapping unit adds an error-correcting code to the stuff present identifier.
In accordance with another preferred embodiment of the present invention, the mapping unit includes supplemental data mapping unit for mapping supplemental information on the sequence of data into free space of the data transmission area of at least one of the plurality of blocks included in each frame. Preferably, the supplemental data mapping unit maps information on the transmission rate at which the sequence of data to be mapped into the plurality of frames in the predetermined form is transmitted, as the supplemental information, into the free space. As an alternative, the supplemental data mapping unit maps information on the status or characteristics of communication upon transmission of the plurality of frames, as the supplemental information, into the free space.
In accordance with another preferred embodiment of the present invention, the mapping unit define both the data transmission area of each of the plurality of blocks into which the sequence of data is mapped and the stuff area in units of one byte so that the sequence of data is separately mapped into the data transmission area of each of the plurality of blocks included in each frame in units of one byte in the same way that the sequence of data to be mapped into the plurality of frames can be separated in units of one byte.
Preferably, the sequence of data is MPEG-2 (Moving Picture Experts Group2) TS (Transport Stream) streams, which are defined by International Standard ISO/IEC (International Organization for Standardization/International Electrotechnical Commission) 13,818-1.
Preferably, the transmission line is a DS3 (Digital Signal, Level 3) network, which is defined by ANSI (American National Standards Institute).
Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings.